In the oil and gas industry, one widely used technique to search for oil and/or gas is to conduct seismic surveys to study subsurface formations. Typically, in seismic surveys, geophysicists use “seismic reflection” techniques to produce an image of the subsurface formations. These techniques generally involve emitting acoustic signals from a seismic energy source that propagate into the earth and recording the signals that are at least partially reflected by the layers of the subsurface formation that have different acoustic impedances. These recorded signals, also called seismic traces, are then processed to render images showing the characteristics or topography of the subsurface area that was surveyed. Typically, processing of seismic traces (raw data) begins with deconvolution, which often improves temporal resolution by collapsing the seismic wavelet to approximately a spike and suppressing reverberations on some field data. In addition, deconvolution also yields a representation of subsurface reflectivity. The next process is conventionally common-midpoint (CMP) or common-conversion point (CCP) stacking, including accompanying processes such as noise attenuation, wavefield separation, multiple attenuation, velocity analysis, normal-moveout (NMO), and statics corrections. Migration is typically a third means to process seismic data. Migration generally corrects and improves initial assumptions that the surveyed formation contains near-horizontal layers. Further, migration is an imaging process that yields a seismic image of the subsurface.
In marine seismic surveying, one method to obtain the seismic data of subsurface formations is ocean bottom recording. Generally, there are two main types of ocean bottom recording: (1) ocean bottom cable (OBC) and (2) ocean bottom seismometer or node (OBS or OBN). In OBC recording, typically, a cable containing multi-component sensors, e.g., geophones and hydrophones, is deployed from a seismic recording vessel to the seafloor using an umbilical connection. The sensors record the seismic data and relay the information to the vessel through the umbilical connection. The spacing arrangement of the multicomponent sensors on the sea floor is similar to that of towed streamers. In OBS acquisition, typically, a number of multicomponent nodes are deployed individually to the sea floor, usually by a remote operated vehicle (ROV). Generally, the typical receiver spacing is between 100 and 400 meters.
In both OBC and OBS acquisition, most modern systems make use of four component (4-C) sensors, consisting of a 3-C geophone and a hydrophone. This allows recording of the full elastic wavefield and separation of the up- and down-travelling waves because hydrophone and geophone respond differently to up- and down-travelling waves.
A hydrophone is a pressure-sensitive seismic detector that is typically used for receivers in marine seismic data acquisition because it enables recording of acoustic energy underwater by converting acoustic energy into electrical energy. Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change. Such piezoelectric materials, or transducers, can convert a sound signal into an electrical signal. A geophone is a velocity-sensitive seismic detector that is typically used for receivers in land seismic data acquisition because it converts ground movement, e.g., particle velocity, into electrical energy—voltage, which may be recorded. The output voltage is proportional to ground velocity.
While the waves propagating through the Earth have a three-dimensional characteristic, geophones are generally constrained to respond to a single dimension, usually in the vertical direction. Some applications may require the recording of the full wave, requiring more specialized equipment such as a three-component (3-C) geophone. Typically, a 3-C geophone comprises three moving coil elements that are mounted in an orthogonal arrangement within a single case. As seen, the combination of both geophone and hydrophone allows recording of the full elastic wavefield and its separation into up- and down-going parts.
Reasons for acquiring OBC or OBS data include the presence of obstructions that make streamer acquisition difficult or impractical, wide azimuth illumination, the ability to also record shear-wave data, a quieter recording environment, higher resolution, among other advantages. Moreover, repeated OBS surveys are an effective tool for time-lapse reservoir monitoring to study the changes taking place in reservoirs over time, particularly producing reservoirs, thereby allowing for the monitoring of fluid, pressure and geomechanical changes. As such, time-lapse seismic imaging of oil and gas reservoirs or “4-D” seismic modeling can provide significant improvements in recovery rates of hydrocarbons and reduce drilling and production costs and risks.
Accordingly, it is desirable to acquire high-fidelity and repeatable 4-D OBS data to reveal subtle changes in the signals which are representative of reservoir changes. That is, 4-D seismic data analysis usually requires high-fidelity seismic data with very high signal-to-noise ratio levels in which, ideally, noise factors are repeatable. As such, it is necessary to minimize all possible differences between surveys that are inevitably caused by changes in acquisition and processing algorithms between vintages of seismic data of the same field or reservoir. Therefore, in 4-D seismic exploration it is desirable to make the acquisition of distinct data vintages as repeatable as possible.
Generally, there are several ways of minimizing these differences. For example, sensor or receiver positions can be made repeatable by permanent deployment of buried hydrophone and geophone sensors at the ocean bottom. Another approach involves using OBN (Ocean Bottom Nodes) which can be positioned very near to the original survey position for subsequent surveys to minimize variations. While receiver-side repeatability may be easily addressed it is extremely difficult to achieve complete repeatability on the source side because of variations in the water column and in the location where the shots are produced. One possible way to address the shot-location problem is to produce the shots in the exact locations or as close to exact as possible. The water column variation problem, however, is more difficult to address because water column variations are caused by many uncontrollable factors, such as tidal effects, sea-water temperature, salinity changes, as well as source-side ghost variations due to variations in sea-surface conditions.
To minimize these water column variations, the current state of the art is to attenuate these undesirable variations with a combination of moveout corrections and static shifts. One example of these methods is disclosed in Lacombe et al., 2009, Correcting for water-column variations, The Leading Edge, pp. 198-201. One disadvantage of these methods is that knowledge of the water velocities and tides is required, and the velocity and tide information is often difficult to obtain. Further, there are other significant drawbacks and limitations to these prior art seismic data processing methods. For example, these corrections methods do not address sea surface variability, e.g., rough sea. In addition, often, a post-stack matching step is required to account for unresolved differences. Further, conventional methods do not address source-side multiples that may vary between surveys due to water-column changes. In general, the current state of the art does not properly account for differences in directivity between surveys. Also, the conventional methods cannot compensate for the difference in subsurface illumination caused by the water column velocity change between vintages. Moreover, these methods cannot be applied to ocean bottom data with sources and receivers located at two different depth levels.
In view of the drawbacks of methods known in the art, there is a great need for seismic data processing to remove water column effect from 4-D ocean bottom seismic data. The present disclosure provides for improved methods and systems that produce high-fidelity and repeatable 4-D ocean bottom seismic data.